For an adhesive to do its job, three conditions must be met in turn: wet the surface, make intimate contact, and cure into a film that can carry load.
1. Wetting
While liquid, the adhesive must spread over the surface. That requires its surface tension to be lower than the surface energy of the substrate. A water drop spreading on clean glass but beading on an oily surface is the simplest example.
- High-surface-energy materials: metals, glass, wood, paper; easy to wet.
- Low-surface-energy materials: PP, PE, PTFE, siliconised surfaces; hard to wet, need pretreatment or special adhesives.
2. Contact
Wetting alone is not enough. The adhesive must penetrate the surface's microscopic roughness with no air gaps between the parts. That is why correct pressure, viscosity and joining within the open time are critical.
3. Curing
The adhesive carries load by turning from liquid to solid, in one of four main ways:
| Mechanism | Example |
|---|---|
| Solidifying on cooling | Hot melts (EVA, PO) |
| Loss of water or solvent | Dispersions, solvent-based adhesives |
| Chemical reaction with moisture | PUR, POR, 1C PU, MS polymer |
| Reaction of two components | 2C PU, epoxy, hybrid |
Why surface preparation matters
Dust, oil, release agents and moisture are the main enemies of wetting. Cleaning, sanding and, where needed, primer or corona treatment decide how well even the best adhesive performs.
With the right adhesive, surface and process, the bond can be stronger than the material itself.
